The present invention relates to a system for remotely measuring the thickness of vessel linings by use of a scanning laser imager. The invention may be used to inspect the interior of refractory-lined vessels such as those used in metallurgical processing and other applications.
The walls of steel furnaces and other vessels used in steel and metal making are typically covered by refractory linings made of bricks. For example, a basic oxygen furnace (BOF) for steel making is typically formed from three shells: an inner working lining of bricks, a middle safety lining also of bricks, and an outer shell usually of steel, and the inner and working linings together are usually about three feet thick. The working lining undergoes uncontrolled and unpredictable wear during steel processing, and to maintain safe and economical production rates, the linings must be only periodically inspected to ascertain their remaining thickness.
Among the systems currently available for measuring the linings of such vessels are those described in U.S. Pat. No. 4,131,914 to Bricmont; and U.S. Pat. No. 4,107,244 to Ochiai et al. These patents describe measurement systems in which a probe, i.e., either a television camera or a microwave sensor, is inserted into the vessel to be measured. The environment within such vessels, which may typically have a temperature of 1,700.degree. C., imposes stringent construction requirements on such sensors. In addition, the vessels to be measured must typically be rotated or otherwise moved to allow the sensors to be inserted. Such motions can interfere with and delay metal processing.
The lining measurement system described in U.S. Pat. No. 4,708,482 to Neiheisel also uses a probe inserted into the vessel to be measured. In the Neiheisel patent, the probe directs a continuous-wave laser light beam at the refractory lining, and a thickness displacement is measured by a self-scanned linear detector array. The signal received by the linear array provides information for mapping the worn or damaged areas of the vessel lining so that such areas may be repaired, for example, by a gunning spray nozzle. Although using a laser, the Neiheisel system suffers from disadvantages similar to those of the foregoing Bricmont and Ochiai et al. systems in that a probe must be inserted into the hot interior of the vessel.
The vessel lining measurements systems described in U.S. Pat. No. 4,893,933 to Neiheisel et al. and U.S. Pat. Nos. 4,508,448 and 4,227,802 to Scholdstrom et al., also use a laser time-of-flight phase measurement technique to determine vessel wall position and thickness, but in these three patents, no probe is inserted into the hot interior of the vessel. In these systems, the vessels are either manually or electromechanically scanned, and each system is carefully aimed at a small number of points at predetermined locations on the vessel surface.
In the Neiheisel et al. system, the apparatus is mounted on a portable cart which can be wheeled into proximity to an open furnace vessel. A laser transmitter directs a laser light beam toward the furnace lining, and the light beam scattered from the vessel is received by a self-scanned linear detector array and correlated by a computer to provide a graphical representation of actual remaining lining thickness. Pneumatically operated locating members cooperate with positioning pads in the floor adjacent the vessel to be measured to accurately and repeatably position the portable cart with respect to the furnace coordinates. The systems described in the two Scholdstrom et al. patents operate in a generally similar fashion.
Since thickness measurements must be made at various times with minimal disruption during the course of a metal making campaign, a useful measurement system must provide data that is located quickly and accurately with respect to the vessel. For example, the Scholdstrom and Scholdstrom et al. systems described above base their lining characterizations on roughly 100 data points that are obtained via a laborious and slow process that is complicated by the necessity of aiming the laser beam in precisely the same directions for each furnace measurement.
In contrast, the present application describes a measurement system employing a scanning light time-of-flight imager and a data processor to enhance the accuracy of the thickness data developed and to reduce the time needed for its collection. The present system is capable of measuring 500,000 range data points for each furnace measurement in seconds and accommodates the finite angular accuracy of its imager by employing a 21/2-D surface-patch modelling process. Even if the prior measurement systems permitted collecting 5000x as many points without lengthening the measurement period impractically, the build up of errors in the measured positions would render the prior systems useless. The present system needs only two minutes for calibrating its position, and collects 30,000 range and reflectance points per second.